If a vehicle suffers from a fault, the vehicle is taken to a repair shop of a dealer or the like. The operator (technician) who is responsible for repairing the vehicle connects an electronic control unit (hereinafter referred to as an “ECU”) on the vehicle to an external diagnosing apparatus, reads fault data (trouble codes) from the ECU, analyzes a defective component or a fault source, and repairs or adjusts the vehicle.
External diagnosing apparatus of the above type usually have an internal power supply. However, certain external diagnosing apparatus exist that are free of an internal power supply for the purpose of making the external diagnosing apparatus smaller, lighter, or lower in cost (see U.S. Pat. No. 5,790,965, hereinafter referred to as “U.S. Pat. No. 5,790,965 A”). According to U.S. Pat. No. 5,790,965 A, an adapter harness 34 of a portable diagnosing apparatus 100 is connected to a connector 2a of an electronic control unit 300 on a vehicle 200. When a power supply switch 35 (FIG. 1) of the portable diagnosing apparatus 100 is turned on, a battery VB on the vehicle 200 supplies electric power to the portable diagnosing apparatus 100 (see column 3, lines 22 through 27, column 4, lines 22 through 29, FIG. 2).